In nature, there exist many systems where some kind of activity propagates without being damped. They are called excitable systems and comprise such different phenomena as spreading of deseases, oscillating chemical reactions, propagation of electrical activity in neurons or heart muscles, and many more (For a review on excitable systems see e.g...

In nature, there exist many systems where some kind of activity propagates without being damped. They are called excitable systems and comprise such different phenomena as spreading of deseases, oscillating chemical reactions, propagation of electrical activity in neurons or heart muscles, and many more (For a review on excitable systems see e.g. [1,2]). Minimize

We investigate a forest-fire model with the density of empty sites as control parameter. The model exhibits three phases, separated by one first-order phase transition and one ’mixed ’ phase transition which shows critical behavior on only one side and hysteresis. The critical behavior is found to be that of the self-organized critical forest-fi...

We investigate a forest-fire model with the density of empty sites as control parameter. The model exhibits three phases, separated by one first-order phase transition and one ’mixed ’ phase transition which shows critical behavior on only one side and hysteresis. The critical behavior is found to be that of the self-organized critical forest-fire model [B. Drossel and F. Schwabl, Phys. Rev. Lett. 69, 1629 (1992)], whereas in the adjacent phase one finds the spiral waves of the Bak et al. forest-fire model [P. Bak, K. Chen and C. Tang, Phys. Lett. A 147, 297 (1990)]. In the third phase one observes clustering of trees with the fire burning at the edges of the clusters. The relation between the density distribution in the spiral state and the percolation threshold is explained and the implications for stationary states with spiral waves in arbitrary excitable systems are discussed. Furthermore, we comment on the possibility of mapping self-organized critical systems onto ’ordinary’ critical systems. Minimize

The question of the existence of order in two-dimensional isotropic dipolar Heisenberg antiferromagnets is studied. It is shown that the dipolar interaction leads to a gap in the spin-wave energy and a nonvanishing order parameter. The resulting finite Néel-temperature is calculated for a square lattice by means of linear spin-wave theory.

The question of the existence of order in two-dimensional isotropic dipolar Heisenberg antiferromagnets is studied. It is shown that the dipolar interaction leads to a gap in the spin-wave energy and a nonvanishing order parameter. The resulting finite Néel-temperature is calculated for a square lattice by means of linear spin-wave theory. Minimize

. -- We discuss the force-velocity relations obtained in a two-state crossbridge model for molecular motors. They can be calculated analytically in two limiting cases: for a large number and for one pair of motors. The effect of the strain-dependent detachment rate on the motor characteristics is studied. It can lead to linear, myosin-like, kine...

. -- We discuss the force-velocity relations obtained in a two-state crossbridge model for molecular motors. They can be calculated analytically in two limiting cases: for a large number and for one pair of motors. The effect of the strain-dependent detachment rate on the motor characteristics is studied. It can lead to linear, myosin-like, kinesin-like and anomalous curves. In particular, we specify the conditions under which oscillatory behavior may be found. Understanding the molecular mechanism underlying biological motors has recently attracted increasing interest in biology as well as in physics [1]. Motor proteins such as myosin, kinesin and dynein moving along molecular tracks are involved in a wide range of processes essential for life, e.g. cell division, muscle contraction, and intracellular transport of organelles. For many decades exclusively data from physiological measurements on muscles [2] provided experimental information for modeling molecular motors [3, 4]. In recen. Minimize

We review our current understanding of the critical dynamics of magnets above and below the transition temperature with focus on the effects due to the dipole–dipole interaction present in all real magnets. Significant progress in our understanding of real ferromagnets in the vicinity of the critical point has been made in the last decade throug...

We review our current understanding of the critical dynamics of magnets above and below the transition temperature with focus on the effects due to the dipole–dipole interaction present in all real magnets. Significant progress in our understanding of real ferromagnets in the vicinity of the critical point has been made in the last decade through improved experimental techniques and theoretical advances in taking into account realistic spin-spin interactions. We start our review with a discussion of the theoretical results for the critical dynamics based on recent renormalization group, mode coupling and spin wave theories. A detailed comparison is made of the theory with experimental results obtained by different measuring techniques, such as neutron scattering, hyperfine interaction, muon–spin–resonance, electron–spin–resonance, and magnetic relaxation, in various materials. Furthermore we discuss the effects of dipolar interaction on the critical dynamics of three–dimensional isotropic antiferromagnets and uniaxial ferromagnets. Special attention is Minimize

We study planar ferromagnetic spin-chain systems with weak antiferromagnetic inter-chain interaction and dipole-dipole interaction. The ground state depends sensitively on the relative strengths of antiferromagnetic exchange and dipole energies kappa=J'a^2c/(g_L\mu_B)^2. For increasing values of \kappa, the ground state changes from a ferromagne...

We study planar ferromagnetic spin-chain systems with weak antiferromagnetic inter-chain interaction and dipole-dipole interaction. The ground state depends sensitively on the relative strengths of antiferromagnetic exchange and dipole energies kappa=J'a^2c/(g_L\mu_B)^2. For increasing values of \kappa, the ground state changes from a ferromagnetic via a collinear antiferromagnetic and an incommensurate phase to a 120^o structure for very large antiferromagnetic energy. Investigation of the magnetic phase diagram of the collinear phase, as realized in CsNiF_3, shows that the structure of the spin order depends sensitivly on the direction of the magnetic field in the hexagonal plane. For certain angular domains of the field incommensurate phases appear which are separated by commensurate phases. When rotating the field, the wave vector characterizing the structure changes continuously in the incommensurate phase, whereas in the commensurate phase the wave vector is locked to a fixed value describing a two-sublattice structure. This is a result of the competition between the exchange and the dipole-dipole interaction. ; Comment: 12 pages, ReVTeX, 13 figures, to be published in Z. Physik Minimize